Patent GB-976376-A (GB)
Title: Device for controlling photographic exposure time
IPC: G03B9/08
Filed: 1962-10-02
Abstract: 976,376. Light modulation; pulse generating Circuits. F. FRUNGEL. Oct. 2, 1962 [Oct. 3, 1961], No. 37248/62], Headings H3P and H4F. [Also in Division G2] A light shutter comprising a Kerr cell 3 located between crossed polarizers 4a, 4b, is actuated by a pulse transformer 14 the secondary winding of which supplies a short duration pulse corresponding to the first differential of the primary current. The arrangement is combined with a light source in the form of a spark gap 10 which is ignited by the discharge from a capacitor 13, the primary of transformer 14 being connected in the discharge circuit. Capacitor 13 is charged from a power source 12. The discharge is effected under the control of a timing device 9, e.g. a quartz crystal oscillator, which applies igniting potentials via a transformer 8 and capacitor 7 to a quenched spark gap device 11 included in the circuit between capacitor 13 and spark gap 10. Spark gap 10 may also have an igniting electrode connected via a capacitor 7 to transformer 8. To ensure that the Kerr cell shutter is actuated at the peak of the discharge via spark gap 10, the pulse from the secondary of transformer 14 is delayed in a delay line 16 which may be arranged to furnish an adjustable delay. 15 comprises a damping resistor. Spark gap 10 is associated with a reflector 6 and illuminates an object placed at 5. Reference is made to the use of short-circuit delay lines connected in parallel with the Kerr cell to limit the duration of operation. The invention may be applied to a conventional type of drum camera in which a film strip is traversed over a circular path. The camera includes an optical shutter (not illustrated) and in order to synchronize the operation of the light source and Kerr cell shutter with the camera shutter the frequency of timing source 9 is either adjusted to match the frequency of the operation of the camera shutter or is controlled by an electro-optical system sensing the passage of perforations in the film strip, the movement of which is linked with the shutter operation. The camera includes an optical system 2 and if desired, the Kerr cell shutter may be located between optical system 2 and the film strip. Reference is made to the use of the invention with ultra-violet light, the nitrobenzene normally employed in Kerr cell 3 being replaced by phenyl-thio-isocyanate and other appropriate changes being made to optical elements. If desired, an ultra-violet to visible light image converter may be included in the system. The use of the differentiation pulse from a transformer to operate the Kerr cell shutter in accordance with the invention permits the attainment of pulses of the necessary power with very short duration (50 manoseconds) and repetition frequencies up to 300,000 per second. The Specification briefly discusses the voltages and powers involved in the arrangement in relation to the parameters of the Kerr cell and also discusses briefly the construction and characteristics of transformer 14. U.S.A. Specification 2,703,374 is referred to in connection with spark gap 10.